U.S. iPhone XR Can't Connect to 4G LTE Signal Abroad In Spain

I had a U.S. iPhone 8 in Spain for over a year and got 4G LTE no problem. It got stolen and when I went back home to the U.S. I got myself an iPhone XR. I didn't change the SIM (which is Orange Mundo PAYG plan if anyone is familiar) and when I returned to Spain I've only been able to connect to 3G and no LTE. I've done all the usual - turn off and turn on data, change around the settings etc but nothing seems to be working. I took it to both the Apple store here and my local cellular service store. The cellular service store said it was a problem for Apple. Apple said I had to ask customer service online but I want to know if anyone else has had a similar issue as well.